Untreated Chlorella homosphaera biomass allows for high rates of cell wall glucan enzymatic hydrolysis when using exoglucanase-free cellulases

BACKGROUND Chlorophyte microalgae have a cell wall containing a large quantity of cellulose Iα with a triclinic unit cell hydrogen-bonding pattern that is more susceptible to hydrolysis than that of the cellulose Iβ polymorphic form that is predominant in higher plants. This study addressed the enzymatic hydrolysis of untreated Chlorella homosphaera biomass using selected enzyme preparations, aiming to identify the relevant activity profile for the microalgae cellulose hydrolysis. Enzymes from Acremonium cellulolyticus, which secretes a complete pool of cellulases plus β-glucosidase; Trichoderma reesei, which secretes a complete pool of cellulases with low β-glucosidase; Aspergillus awamori, which secretes endoglucanases and β-glucosidase; blends of T. reesei-A. awamori or A. awamori-A. cellulolyticus enzymes; and a purified A. awamori β-glucosidase were evaluated. RESULTS The highest initial glucan hydrolysis rate of 140.3 mg/g/h was observed for A. awamori enzymes with high β-glucosidase, low endoglucanase, and negligible cellobiohydrolase activities. The initial hydrolysis rates when using A. cellulolyticus or T. reesei enzymes were significantly lower, whereas the results for the T. reesei-A. awamori and A. awamori-A. cellulolyticus blends were similar to that for the A. awamori enzymes. Thus, the hydrolysis of C. homosphaera cellulose was performed exclusively by the endoglucanase and β-glucosidase activities. X-ray diffraction data showing negligible cellulose crystallinity for untreated C. homosphaera biomass corroborate these findings. The A. awamori-A. cellulolyticus blend showed the highest initial polysaccharide hydrolysis rate of 185.6 mg/g/h, as measured by glucose equivalent, in addition to the highest predicted maximum glucan hydrolysis yield of 47% of total glucose (w/w). T. reesei enzymes showed the lowest predicted maximum glucan hydrolysis yield of 25% (w/w), whereas the maximum yields of approximately 31% were observed for the other enzyme preparations. The hydrolysis yields were proportional to the enzyme β-glucosidase load, indicating that the endoglucanase load was not rate-limiting. CONCLUSIONS High rates of enzymatic hydrolysis were achieved for untreated C. homosphaera biomass with enzymes containing endoglucanase and β-glucosidase activities and devoid of cellobiohydrolase activity. These findings simplify the complexity of the enzyme pools required for the enzymatic hydrolysis of microalgal biomass decreasing the enzyme cost for the production of microalgae-derived glucose syrups.